It is important to set suitable values of electric currents, select suitable positions of electrodes and, especially, position the electrodes perpendicularly to portions of workpieces to be subjected to spot welding, so as to provide high quality of spot-welded portions of the workpieces. Positioning jigs have been used in positioning electrodes for spot welding, as disclosed in, for example, JP-A-2004-82168. The disclosed positioning jig for use in positioning an electrode will be discussed with reference to FIG. 13 and FIG. 14 hereof.
FIG. 13 illustrates the conventional positioning jig designated at 100 for positioning an electrode 105 for spot welding. The jig 100 includes an attaching portion 103 for magnetically attaching to a surface 102 of a flat workpiece 101, and a bar-shaped positioning portion 104 extending upwardly from the attaching portion 103 and disposed perpendicularly to the surface 102 of the workpiece 101.
The positioning portion 104 has a lower part spaced an interval Y1 from the electrode 105. The positioning portion 104 has an upper part spaced an interval Y2 from the electrode 105. When the interval Y1 is found to be equal to the interval Y2 through comparison of the interval Y1 with the interval Y2, the electrode 105 is disposed in parallel to the positioning portion 104 and therefore the electrode 105 is found to be appropriately positioned. When the interval Y1 is different from the interval Y2, the electrode 105 is inclined relative to the positioning portion 104. Therefore, the inclination of the electrode 105 is corrected to make the interval Y1 equal to the interval Y2.
When the workpiece 101 is curved, as shown in FIG. 14, however, the electrode 105 is not disposed in parallel to the positioning portion 104. In this case, if a posture of the electrode 105 is adjusted to make the interval Y1 equal to the interval Y2, the electrode 105 is not disposed perpendicularly to a portion of the workpiece 101 to be subjected to spot welding. With the electrode 105 disposed not perpendicularly to the portion of the workpiece 101, spot welding can not be satisfactorily performed on the portion of the workpiece. For the positioning jig 100, therefore, it is impossible to appropriately evaluate a posture of the electrode on the basis of the intervals Y1, Y2 when the workpiece is curved as shown in FIG. 14.
To address the above problem, there is a demand for a technique capable of positioning an electrode perpendicularly to a portion of a workpiece to be subjected to spot welding, regardless of whether the workpiece is flat or curved.